Compressed video signal transmission systems, for example systems using an MPEG2 (Motion Picture Experts Group) compression format (“Coding of Moving Pictures and Associated Audio,” ISO/IEC JTC1/SC29/WG11 N0702 (revised), May 10, 1994), are currently broadcasting HDTV (High Definition Television) digital signals from a number of test sites. Commercial program broadcasts are scheduled to begin soon as the first HDTV sets come to market. HDTV signals are not compatible with current television receivers, such as those for processing NTSC standard signals in the United States. Therefore, a transition period will occur during which SD (standard definition) television signals according to the NTSC or PAL television standards will continue to be broadcast so as to prevent SD sets from becoming immediately obsolete. Also for a period of time, some programming will not be available in MPEG2 format because of changeover logistics broadcasters will encounter.
Video data is transmitted in different formats (e.g. 4:3 and 16:9 picture display aspect ratios; 4:4:4, 4:2:2, and 4:2:0 data sample formats; interlaced and non-interlaced scanned) and with different spatial resolution (e.g. 352, 480, 544, 640, 720 . . . 1920 pixels per line and 240, 480, 720, 1080 active lines per frame). It is generally impractical, for both aesthetic and cost reasons, to equip video signal receivers with the ability to display decompressed signals in their pre-transmission format. Rather, post decompression processing circuitry is preferably included to transcode different formats of a decompressed video signal to a desired display format.
There are many transcoding or spatio-temporal conversion systems known to those skilled in the art of video signal processing. In general, each is directed to a specific type of conversion, such as interlace to non-interlace conversion, or sample, line, or field rate doubling.
Even though video decompression systems incorporate an appreciable amount of circuitry, it is desirable to employ additional circuitry to process non-compressed or standard definition video signals. Post processing circuitry included in the receiver should transcode an SD video signal without significantly increasing the amount of transcoding circuitry. This is difficult because digital MPEG2 formatted television signals arrive at an MPEG2 compatible display processor in decoded pixel block format. SD television signals generally arrive at the display processor as multiplexed analog YCRCB display-ready pixel lines (a raster scan) in a 4:2:2 ratio in either NTSC or PAL format. Also, SD signals are of lower resolution than many of the high definition (HD) displays associated with MPEG2 HD signals. Upconversion which correctly compensates for motion occurring in an image is a complex process because the image is presented temporally as interlaced field data. Significant memory is required to construct an image frame suitable for display.